In today's electrical supply systems, there are occasions when alternate sources of electrical power are necessary or desirable. For example, the capability of switching from utility power to emergency generator power is extremely important for many businesses, hospitals and industries, as well as residential dwellings.
In certain applications, it is desirable for separate electrical circuits or even separate groups of electrical circuits to be arranged so that when one group of circuits is switched to a conductive state, another group of circuits is switched to a non-conductive state in an alternating fashion. In addition, it may be desirable to alternately switch a common load between separate power sources, so that as one power source is disconnected from the load, the second power source is connected after a negligible delay so as to limit interruption of electrical power to the common load. In order that the desired delay in alternate switching between power sources may be minimized, a need has been recognized to employ an interlock mechanism or assembly which functions to switch one group of circuits OFF as the other group of circuits is switched ON.
A certain known transfer-type electrical panel of a building, typically located adjacent to the service entrance electrical panel, includes a pair of transfer-type switches that selectively control the supply of electrical power from either a standard utility 125/250 VAC service or a generator power supply. This known type of transfer arrangement controls the supply of electrical power from the two “hot” conductors of the generator. The neutral conductor from the generator is directly connected to the neutral of the building electrical system, and the safety grounding conductor is bonded to a neutral bus at the service entrance panel. This system configuration is commonly referred to as a “non-separately derived system.” The typical generator is a single-phase 125/250 VAC “floating-neutral” generator that includes an electrical outlet configured to provided two “hot” legs, a neutral, and a safety grounding conductor. A characteristic of the “floating-neutral generator is that the neutral conductor and the safety ground conductor are not bonded together.
There are instances in which it is desirable to use a 125/250 VAC “bonded-neutral” generator (which includes a neutral conductor and a safety ground conductor that are internally bonded together) for the purpose of powering structures or dwellings. A building is typically fed by a standard utility 125/250 VAC service that includes a neutral bus conductor connected to a safety ground bus conductor, and the safety ground bus conductor connected to a grounding rod or net. Using the non-separately derived configuration described above, the pair of “hot” conductors from the generator are connected to the appropriate poles of the two-pole transfer switch, the neutral conductor of the generator is permanently connected to the neutral bus conductor of the electrical panel, and the safety ground conductor of the generator is permanently connected to the safety ground bus conductor of the electrical panel.
However, this configuration has drawbacks when used with bonded-neutral generators. For example, assume the transfer switch of the above-described system configuration is in the ON position such that the generator is supplying electrical power via the pair of “hot” conductors to a common load in the building. Electrical current flows from one of the “hot” conductors of the generator through the transfer switch and through a conventional distribution breaker at the electrical panel of the dwelling in a known manner so as to power the electrical load in the building. The electrical current then returns via the neutral conductor of the load to the neutral bus conductor of the electrical panel. A first portion of electrical current then flows from the neutral bus conductor of the electrical panel back to the neutral conductor of the generator, thus completing the circuit path. A remaining portion of electrical current flows from the neutral bus conductor of the electrical panel to a neutral-to-ground tie bar at the electrical panel, through a ground bus conductor, back through the safety ground-to-neutral bonding conductor of the generator, and then through the neutral conductor of the generator, completing another circuit path. It is this undesired dual path for electrical current to follow back from the electrical load to the generator that creates a problem.
Rather than the dual path current flow described above, such a power system should be electrically grounded in such a manner that prevents a flow of electrical current via the neutral conductor of the building back to the safety ground conductor of the generator, in all situations except for an electrical power fault (q.v., Article 250 of the National Electrical Code). The safety ground conductor is expected to be pristine or absent of the normal flow of electrical current, and instead is to be used to conduct electrical current safely to ground only when there is an electrical fault occurrence. Thus, known system configurations are undesirable because such configurations allow a normal flow of electrically current to pass via the neutral conductor of the building to the safety ground conductor of the generator. Another drawback of above-described system configurations is that the flow of electrical current to the safety ground conductor of the generator is known to trigger a ground fault circuit interrupt at the generator. When triggered, the ground fault circuit interrupter will de-energize the “hot” conductors of the generator and prevent the supply of electrical power to the service bus conductor of the electrical panel.
In an attempt to address the drawbacks described above, a “separately-derived” system configuration can be employed. This system configuration uses a transfer switch arrangement that makes or breaks the neutral conductor as well as the two “hot” conductors of a “bonded neutral” generator. Again, for purposes of example, assume the transfer switches are initially positioned such that electrical current flows from one of the pair of “hot” electrical conductors of the generator to the common load of the building. Specifically, the electrical current flows from the “hot” conductors of the generator through the transfer switch in a known manner, and to the electrical load. The electrical current then returns via the neutral conductor of the electrical load. However, instead of electrical current flowing through the neutral bus conductor of the electrical panel, the flow of electrical current is routed by a separate neutral switch assembly to the neutral conductor of the generator, thus completing the circuit. Thereby, this system prevents the undesired flow of electrical current through the generator safety ground-to-neutral bonding conductor and back to the generator neutral conductor, as noted previously.
However, this known system configuration also has drawbacks, specifically involving the switching sequence of the neutral transfer switch assembly that controls the electrical connection of the neutral conductor of the utility service or generator with the neutral bus conductor of the building. In the switching sequence, there is a potential to execute an “open-neutral connection” switching event, which can occur when the transfer switch establishes connection of the “hot” conductors of the generator or utility service to the service bus conductor of the building before the transfer switch establishes connection of the neutral conductor of the generator or utility service to the neutral bus conductor of the building. An “open neutral” condition such as this may last for only a short period of time. Given that each of the operating handles (i.e., the handle interconnecting the switch for each “hot” conductor connection, and the handle for operating the neutral switch for the neutral connection) of the transfer switch are mechanically connected together, this system configuration can increase the delay or lag time between actual connection of the electrical contacts at each of the switched poles. In an open neutral switching event, the path for electrical current to return back via the neutral connection at the transfer switch assembly is interrupted for a short period of time. However, there is a complete circuit path for electrical current to flow from one “hot” conductor of the generator to the other. In this event, electrical loads that are normally connected in parallel can be connected in series. This series connection of electrical loads results in the same electrical current draw through each electrical load, causing much larger voltages to be experienced by the electrical loads. Under certain conditions, this can be the equivalent to plugging a standard 120-volt appliance into a 240-volt outlet, causing an undesirable over-voltage condition at the load. The lag time in closing the electrical contact of certain commonly used molded-case circuit breakers and switches can result in an “open neutral” connection switching event that can last as long as 10 milliseconds, increasing the statistical probability that an open-neutral switching event will occur at a voltage maximum that can result in the over-voltage condition at the load.
Another certain known system configuration uses an “overlapping-contact” transfer switch to individually control interruption of the neutral connection to the generator or the utility service at the electrical panel. This overlapping-contact transfer switch configuration establishes the neutral connection to the generator or utility service before breaking or interrupting the electrical connection to the neutral bus conductor of the building. However, the drawback of this system configuration is that the electrical connection of the neutral conductor of the generator with the neutral conductor of the utility service results in the flow of electrical current through the generator safety ground, which is undesired for reasons described above.
Therefore, there is a need for a transfer switching device that can be operated by a single mechanism to provide sequenced transfer switching between power supplies in an electrical panel, including neutral connection switching. There is also a desire for a interlock assembly configured to create the following switching sequence: interrupt connection of the presently connected “hot” conductors of a first power supply to the service bus conductor, then interrupt connection of the neutral conductor of the first power supply to the neutral bus conductor, then establish connection of the neutral conductor of the second power supply to the neutral bus conductor, and then finally to establish connection of the “hot” conductors of the second power supply to the service bus conductor, and vice versa. In this manner, at no time are the neutral conductors of the power supplies connected together. Furthermore, at no time are the “hot” conductors of either of the power supplies electrically connected to the service bus of the electrical panel without the associated neutral line conductor having been connected in advance to the neutral conductor of the electrical panel.